Inductor



March 12, 1957 w. w. sALlsBURY INDUCTOR 2 Sheets-Sheer?. l

Filed Dec. 5, 1952 March 12, 1957A w. W. SALISBURY 2,785,265

INDUCTOR Filed Dec. 5, 1952 2 sheets-snee@ 2 ATTORNEYS United States Patent OT INDUCTOR Winfield W. Saiishury, Lafayette, Calif., assigner to Zenith Radio Corporation, a corporation of iiiinois Application December 5, 1952, Serial No. 324,199

3 Claims. (Cl. 219-10.79)

This invention relates to apparatus for producing high frequency magnetic fields at very high intensity, i. e. of the order of 500,000 gausses or more, for induction heating or for other purposes where elds of this density are useful; specifically, for the production of high intensity lillumination from extremely small sources, approaching, in size, the theoretical point source.

The application of high frequency fields of moderate intensity for induction heating is well understood. The production of elds wherein the liux density rises to such values as are here contemplated, however, becomes quite diihcult. To produce ux densities of this order of inagnitude requires a very large number of ampere turns; if these are provided by coils of many turns and high inductance t-he voltage necessary to cause the required currents to dow becomes high, insulation problems arise, and the distributed capacity of the coil, in connection with the high inductance, limits the frequencies which can be effectively employed. lf the large number of ampere turns is provided by decreasing the number of turns and increasing the current, the inductance of the leads becomes a dominating factor, and the transformation losses involved in stepping down the high voltage outputs, developed by substantially all high frequency generators, also enter the picture. High frequency transformers of the air core type usually have a large leakage reactance which also tends to limit the current available at the coil itself.

The primary object of the present invention is to minimize the diiculties mentioned. To this end, among the objects of the invention, are to provide a low impedance inductor wherein substantially the entire effective inductance is concentrated in the coil itself; to provide an inductor which will produce magnetic fields of one-half `million or'V more gausses ux density in an extremely limited volume; to provide a means for producing inagnetic fields of high density at the desired point of application, but wherein the flux density falls off with extreme rapidity with distance from a point where the flux is to be applied; to provide an inductor of the character mentioned which will utilize eiiiciently energy supplied at frequencies in the megacycle range; to provide circuits, for supplying such an inductor, of high input and low output impedance and inherent low leakage reactance; to provide a type of inductor and associated supply line 'wherein the transition between the l-ine and the inductor Vthe type herein considered is primarily one of getting power to the point where it is to be utilized with maximum efficiency. Attenuation between a power source and a useful'load may be caused by relatively high series impedance (almost always inductive), or by capacitive admittance in shunt with the load. Ordinarily both sources of loss are involved to some extent. The higher 2,785,26 Patented Mar. 12, 1957 the series impedance the greater the potential that must be applied to cause a given current to ow, and the higher the potential the greater will be the loss through stray capacities between the leads of the transmission lines supplying the linductor or elsewhere. Ordinarily this problem has been attacked by an attempt to minimize the leakage and to increase the supply voltage to a greater or iess extent. In accordance with the present invention a directly contrary approach is used and this involves the paradoxical problem of providing an inductor having minimum inductance, both intrinsically and in its supply leads.

Broadly stated, the apparatus of this invention cornprises an inductor formed of an edge-wise wound at conductor, the width whereof Kis large in comparison with its thickness. The conductor is formed into a coil which, for most purposes, is preferably a single turn and for substantially all purposes comprises very few turns. The inside diameter of the coil is small in comparison with the width of the conductor which forms it; it is undesirable to have the inside diameter more than one-tenth -the width of the strip forming it and the inside diameter may be only one one-hundredth of the conductor width. The coil thus formed is continued as a pair of leads which may conveniently be formed integrally with the coil in any event should be similar in cross-section, i. e., the width much greater than the thickness and of generally rectangular cross-section. These leads are superposed, one above the other, in parallel planes and juxtaposed as closely as is possible without electrical Contact between them. lf desired they may be separated by a thin layer of insulation, such as mica or one of the low-loss plastics such as polystyrene sheets, but because of the lc-v/ voltages employed varnished carnbric or even untreated paper may be used. The leads connect directly to a single-turn secondary of a radio-frequency transformer, the primary whereof has the required number of turns to give a stepdown ratio matching the impedance of the inductor with the impedance of the source from which it is to be supn plied. The coupling between primary and secondary cf the transformer should be as close as is mechanically and electrically possible, if the ultimate advantage obtainable from the invention is to be achieved and this can best be accomplished by a device wherein the primary winding is an air core toroid comprising two equal coils in parallel, each coil being of a single layer and the two coils being Wound in opposite direction and so disposed that each en- Compasses one half of the circumference of the toroid. This Winding is surrounded by a case which forms a singleturn secondary winding in conjunction with a central conductor, extending through the hole in the toro-idal primary and connecting the two ends of the outer casing. The latter is split at some plane parallel to the major plane of the 'toroid itself to form a peripheral gap, on either side of which the two leads from the inductor are connected.

When the arrangement as thus described is excited from a suitable high frequency source the high intensity field is generated within the central opening of the inductor. From this opening the lines of force spread out nearly radially, so that the field attenuates with great rapidity. Any object placed within the central opening which is capable of inductive heating will be raised very rapidly to a high temperature, but the heating will be localized almost entirely in the plane of the conductor. To use this heating to form a high intensity light source of small size the tip of a conducting rod of high melting point material may be inserted at the center of the opening. The preferable material for this purpose is tungsten. The tip of the rod will fuse almost instantly, but the heat is so highly localized that the molten portion will adhere and the temperature canbe raised to a point equaling or exceeding that of an arc crater. By owing a non-oxidizing gas,

- avances such as hydrogen, helium, or carbon dioxide around the hot tip of the rod it can be prevented from burning and the loss from the tip limited to vaporization. By making the rod of small size there thus becomes available a very high intensity source which, because of its minute dimensions, may be focused into a beam having avery narrow angle of spread.

All of the above will be more readily understood by reference to the ensuing description of preferred forms of the invention, together with the accompanying drawings wherein:

Fig. 1 is -a plan View of an induction heater in accordance with the invention;

Fig. 2 is `a side elevation of the appar-atus of Fig. l;

Fig. 3 is ya cross-'sectional View of the radio frequency transformer Iillustrated in Figs. l and 2;

Fig. 4 is an elevation of the inductor of this invention together with the rod used for generating a point source light and the supply for a non-oxidizing gas;

Fig. 5 is a schematic diagram of the system as illustrated in Fig. 1;

Fig. 6 'is a perspective showing of the inductor of this invention as used with a conventional type of high frequency transformer;

Fig. 7 is a fragmentary end elevation of the inductor of Figs. 1 and 6;

Fig. 8 is an isometric View showing a modified form of inductor.

Figs. 1 'to 3 show va preferred form of the invention. The inductor proper is a single edgewise wound turn 1 of fiat conductor. Since the entire principle of oper-ation of the device is based on providing an inductor of -minimum impedance, the material should, of course, 'be as highly conductive 'as possible; silver, copper, or aluminum. Since the device finds its maximum utility at very high frequencies, where skin effects are pronounced, as well as for other reasons later to 'become apparent, maximum surface larea is desirable 'and the dimensions of the strip are such as -to provide the necessary effective cross-section to carry the required currents with as thin a conductor as possible. The width -of the conductor should therefore be large in comparison with its thickness and it is generally preferred to make the latter merely sufhcient to give the necessary mechanical rigidity; the thickness is exaggerated in the drawing in order to show the construction more clearly. The inside diameter of the single-turn coil thus formed around a central opening 2 is also small in comparison to the width of the conductor. For the uses for which the invention is primarily designed it will usually not be greater than one-tenth of the conductor width and it may be as little as one one-hundredth of this width.

Electrically continuous with the coil 1 are leads 3 and 3' which are of wide, flat, metal sheet or strip generally similar in cross-section to the coil itself. These leads are brought out closely superposed in parallel planes, with their adjacent faces in as close juxtaposition as is possible while maintaining electrical insulation between them. The efective voltage Iacross the leads will be relatively very low, and if the strips are suiciently rigid so that positive separation can be maintained between them `air dielectric may be sufficient. Usually, however, it is desirable to provide a thin layer of solid dielectric .5, which may be fibrous, plastic, or -even lacquer applied to the conductor. The leads connect to the secondary coil of a transformer, generally designated by lthe reference character 7.

While the coil 1 has been referred to as edgewise wound" this refers to its shape and not its method' of formation. The coil and its connecting leads 'are most easily constructed of two similar pieces of sheet metal, cut to the shape shown and silver soldered or brazed together Ialong the seam 4.

The primary 9 of transformer 7 comprises two oppositely wound, single layer coils in parallel, each coil factor.

embracing one-half of the major circumference of la toroid. Such a coil may 'be wound on a suitable mandrel (in this case square) starting at the center `and winding both strands at the same time -outwardly from the central point. When the proper number of turns has been wound on each half, an insulated supporting strip 11 may be cemented to one side of the coil and the mandrel withdrawn. Bending Ithe strip 11 into a circle brings the two outer ends of the coils together to forma complete toroid, with taps or connections at the two opposite sides thereof. The mounting strip 11 is preferably a low-loss plastic, such as polystyrene.

The transformer secondary is a toroidal casing enveloping the primary substantially completely. Normally it is made of copper, Iand comprises a central tube 13 extending axially through the cylindrical casing 15. The tubular centr-al conductor 13 is, in this case, slightly longer than the side wall 17 of the casing. A cover plate 19, electrically continuous with the upper lead 3, is secured to and electrically connected with the central conduct-or 13, and completely encloses the primary except for a Vnarrow peripheral gap between the -cover plate and `the side wall 17 of the container.

The insulating strip 11 is cemented or otherwise secured to the central conductor, to hold the primary coil 9 in place Iand equally spaced from the enveloping secondary coil on all sides. Additional spacing rings 21, which may be of the same material las the strip 11, maybe used between the primary and the upper `and lower circular covers of the secondary if the size of the structure is such as to make additional support desirable, `but the amount of solid dielectric in the structure is preferably kept lat Va minimum in order to reduce both distributed capacities and dielectric losses. The lower lead 3' is connected to the upper rim of the side w-all 17 as is best shown in Fig. 2. The primary leads are brought out through suitable apertures 23 on opposite sides of the side wall.

In order to appreciate the benefits of the structures thus described 'a consideration of some theoretical factors is necessary. Nearly all high frequency devices, particularly vacuum tubes of various types, have a high internal impedance and therefore ou-tput circuits in general are designated to match this impedance. The loads, however, are quite generally either inductive or resistive; where they are inductive their frequency of operation is usually limited by stray capacities of various kinds. The distributed capacity of coils, capacity between leads, and capacity from leads to ground are therefore usu-ally considered to be necessary evils and every effort is made to minimize them.

It is not, however, the numerical value of capacitance in la circuit which is important or deleterious but its relation to the other circuit constants. From the standpoint of frequency response capacity can be increased indefinitely if the inductance can be decreased by the same or a greater factor. From the aspect of transmission-line attenuation the loss through interlead capacity is proportional to the square of the potential across the leads and to the rst power of the capacity. Again it comes out that an increase in capacity does not harm if the total series impedance of the load can be reduced by an equal It follows that if series impedance can be reduced by a greater factor than the increase in shunt capacity there will be a net over-all gain.

In a device the primary purpose of which is to produce a concentrated high-flux density magnetic field there must be series inductance in the circuit. The fundamental basis of the present invention is to make substantially all of the circuit inductance elective in the production of the useful lield. In order to achieve the very large number of ampere-turns required to produce elds having the ux densities of the order that has been mentioned the large number of ampere-turns required is achieved by using the minimum possible number of turns-namely,

out.

Y creased by decreasing the spacing.

arcanes one--and increasing the amperage to the necessary value.

In all formulas for the computation of inductance the shape of the conductor enters in the form of a factor which is proportional to the logarithm of the length of the conductor divided by the periphery; the diameter in -given cross-section, the wider and thinner the conductor -used the less will be its inductance.

For the purpose of the present rinvention the axial dimension of the coil should be as small as possible in any event, but it also comes out that this will, in general, lead to a minimum inductance. The greatest single gain of the present construction lies, however, in the way'the leads are brought The effective inductance of these leads varies as an inverse function of their spacing, and with wide flat leads not only is the inherent inductance of the leads reduced to a minimum but total effective inductance is further de- If the outgoing and incoming leads could be made to occupy the same physical space their mutual inductance would exactly counten balance their self inductance and the net total inductance of the transmission line would be zero. The hat, closely adjacent leads come as near as is physically possible to accomplishing this effect; the reduction in effective inductance of the circuit by reason of the form and position of the leads is by a much greater factor than is the increase in capacity and the result is a large gain in both frequency response and in the percentage of generated energy delivered at the point of application.

' If a coil of more than one turn is used the inductance varies substantially as the square of the number of turns, the necessary current to produce a given field varies invversely as the turns and the voltage to produce this current varies directly as the turns. The energy required at the coil terminals remains the same, but if the spacing of the leads isr kept the same the maximum frequency at which the'device is effective varies inversely with the turns and the percentage loss through lead capacity rises directly. Moreover, distributed capacity is introduced in the coil itself. If the leads are separated to cut down their capacity the series inductance rises. Therefore, while certain of the advantages of the invention may be realized by using a coil of more than one turn, a single turn coil gives optimum results and is preferred unless considerations not directly related to the invention per se are controlling.

One example is where a uniform high intensity field of greater depth is required. In this case a coil of two turns may be used, the turns being spaced axially by a distance equal to the inside radii of the turns to form a Helmholz coil.

The form of transformer used to supply the inductor is designed to carry out the same fundamental principles of high-current, low inductance and low over-al1 series impedance. The fact that available sources of high frequency are substantially all of high internal impedance has already been referred to, and for this reason equipment for stepping down voltage and stepping up the current is necessary. In order to match the impedance of the generator, the inductance of the primary must be relatively high and therefore distributed capacity assumes its more customary roles in the limitation of frequency response and as a cause of energy loss in the transmission. Transfer of energy from primary to secondary is limited by the leakage reactance of the transformer, and therefore its coeicient of coupling should be as high as possible. Toroidal coils have long been used in the radio frequency art. In general, however, they have been made of a coil originally wound as a solenoid and bent into a circle with the two ends immediately adjacent, so that the capacity between the end turns has maximum effect. With a primary coil wound as is here shown, as two oppositely wound halves in parallel,

the adjacent end turns are at the same potential and do not contribute to the distributed capacity nor is there any tendency to spark between them at high applied voltages; the potential drop is uniformly distributed along the coils. In low frequency apparatus, Where ferro-magnetic cores may be used, it is not difficult to devise a transformer having a coefficient of coupling approaching unity. Where the frequencies employed are so high as to make air core transformers practically mandatory it is much more difficult to get high coupling coefficients. Using a single turn secondary, the latter, as viewed from the primary, may be considered as substantially a uni- Y potential surface and its presence `contributes to the distributed capacity which limits the frequency response of the primary coil. If the coupling coefficient is to be highthis is true substantially irrespective of the form taken by the primary and secondary coils.

It would be possible to use, with an inductor of thc type here described, a secondary winding such. as that illustrated at 15', Fig. 6 surrounding a solenoid primary 19 of like length. With equal spacings between primary and secondary, and equal inductance in the primary coil, the effective distributed capacity added would be the same with either form of transformer. The leakage reactance in the transformer of Fig. 6, would, however, be much greater. It is known that a coil of toroidal form has no external field other than that of a single turn of wire in the major plane of the foroid. With the form of primary here shown, even this field is eliminated and the primary field is limitenl to that space which is fully enclosed by the winding. T he same holds true of the single turn secondary and the coeicient of coupling of the device .is equal to the volume within the primary divided by the volume in the secondary. A feature of the transformer here shown is that the secondary coil encloses and shields the primary completely, giving a maximum coefficient of coupling for a given effective distributed' capacity and therefore reduces to a minimum parasitic impedance or leakage reactance of the device. Transformers of this type have been constructed with a coefficient of coupling of approximately percent, a value which cannot be approached with ordinary air core structures. Therefore, while the type of inductor and transmission line here disclosed may be used with a transformer such as is shown in Fig. 6 with a considerable advantage over an inductor of the ordinary type, that advantage is multiplied by supplying it from a transformer such as is shown in the first three figures.

It should, of course, be evident that it is not necessary that toroidal coils of square section be used; a toi-oid of the traditional doughnut shape will also serve and, in fact, give slightly improved efliciency, but its manufacture is not quite as simple as it is where a rectangular coil section is used.

It should also be mentioned that it is not necessary to bring in the secondary leads between one cover and the top of the side wall as is here shown. The split in the case where the leads join may be around any circumference of the side wall, as long as the leads are brought in in their closely parallel arrangement.

While the transformer-inductor combination described will usually be used with a high frequency source of continuous oscillations it is also operative from a pulse source, such as a charged condenser or transmission line. Such a source may be charged to a high voltage and discharged through the transformer primary. It is easy to construct a transformer of the type described with effective ratios from 30:1 to 100:1. With the latter ratio a condenser of 2.5 mf., charged to 1000 volts, will hold 2.5 millicoulombs and will discharge in about 4 microseconds, giving an average primary current for this period of 62 amperes or a secondary current of 6250 amperes. This high current accounts for the very high field available.

It should be evident that it is not necessary that the leads supplying the coil be brought out in the same plane as the coil itself. Fig. 7 shows a modied form of inductor wherein the coil 27 is formed of two semicircular hook shaped ends formed integrally with the wide straps 29 which form the leads. The latter are bent at right angles to the hooks and brought closely together, separated by a layer of insulation 31, and the two halves of the coil are brazed or welded along the radius 33. Various other equivalent structures are possible which will give the characteristic wide iiat coil and minimum inductance leads of this invention.

Fig. 4 shows the application of the device to produce a high intensity light from practically a point source. The coil 1 with its leads 3 is shown edge on. coil is mounted a supply pipe 43 through which helium, hydrogen, or carbon dioxide may be fed. Extending upward from a T 45, which terminates the pipe 43, is a short section of insulating tubing 47, terminating immediately below the inductor 1. A gland 49 in the bottom of the T-connection admits a thin tungsten rod or wire 51, which passes up through the insulating tube 47 and into the central aperture 2 of the inductor.

The tip of the rod 51, lying within the highly concentrated iield, is fused almost immediately, but because of the radial spreading of the lines of force immediately beyond the plane of the inductor, the heating is highly localized and it is only the tip which melts. Owing to the small diameter of the rod, longitudinal thermal conduction along it is relatively slow. Furthermore, the surface area of the rod, because of its small diameter, is large in comparison with the volume and therefore loses heat rapidly, both by radiation and by convection to the gas flowing past it. Because the gas is inert it prevents burning of the tip and since it may be operated at atmospheric pressure there is not as much loss by evaporation as there would be in vacuo. With the rod vertical a small molten globule at its end clings to the tip and since the temperature is very much higher than that at which tungsten, or, in fact, any of the metals, can normally be operated, the light intensity per unit areai. e., the intrinsic brilliancy-can be made very great, equalling or exceeding that of an arc crater and having the additional advantage that it is unobscured by an opposing electrode. Furthermore, because of its small size, approaching the theoretical point source, it can be focused into a beam subtending a very small angle and can be utilized in the many Beneath the types of optical equipment where such a narrowbeam is desired.

Having thus described the invention, what is claimed is as follows:

1. An inductor for producing concentrated high frequency magnetic elds comprising an edgewise-wound conductor of substantially rectangular cross section, the width of said conductor being materially greater than the thickness thereof forming a coil the inside diameter whereof is small in comparison with the conductor width, and mutually insulated leads of generally similar cross section to said coil for supplying the same, said leads being superposed in parallel closely adjacent planes to minimize the effective inductance thereof.

2. An inductcr as dened in claim 1 wherein said coil comprises a single turn.

3, An inductor for producing intense localized magnetic elds comprising a pair of superposed strap leads of sheet material each having a Width atleast an order of magnitude greater than its thickness, at least one of said leads being formed with a hooked end bent out of the general plane of the lead of whichit is a part to abut the end of the other of said leads, a metallic bond joining the abutting ends of said leads to form a at single turn coil having an internal diameter small in comparison with the width thereof, and a layer of insulation interposed between the superposed leads.

References Cited in the tile of this patent UNITED STATES PATENTS 558,634 Colby Apr. 2l, 1896 1,478,262 Snodgrass Dec. 18, 1923 1,684,134 Rentschler Oct. 26, 1926 1,691,699 Cardwell Nov. 13, 1928 2,181,899 Kennedy Dec. 5, 1939 2,314,865 Bierwirth Mar. 30, 1943 2,544,845 Link Mar. 13, 1951 2,587,169 Kivari Feb. 26, 1952 2,604,519 Mackereth July 22, 1952 2,676,233 Foxx Apr. 20, 1954 2,676,296 Sabol Sept. V14, 1954 FOREIGN PATENTS 309,211 Germany Sept. 22, 1919 655,400 Great Britain July 18, 1951 680,140 Great Britain Oct. 1, 1952 997,266 France Ian. 3, 1952 

